Final answer:
The apt upgrade command is used in Debian-based systems to update all installed packages to their latest available versions from the configured repositories.
Step-by-step explanation:
The command to install the latest versions of all packages currently installed on the system from the sources configured in sources.list on a Debian-based system like Ubuntu is apt upgrade. When you run this command, the system will first check for updates with apt update to refresh the package list, then apt upgrade will download and apply the updates. Be mindful that this will not install new packages or remove existing ones, but it will update all installed packages to their latest versions available from the repositories.